US8174803B2 - System for creating a magnetic field via a superconducting magnet - Google Patents

System for creating a magnetic field via a superconducting magnet Download PDF

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US8174803B2
US8174803B2 US12/742,296 US74229608A US8174803B2 US 8174803 B2 US8174803 B2 US 8174803B2 US 74229608 A US74229608 A US 74229608A US 8174803 B2 US8174803 B2 US 8174803B2
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limiter
current
magnet
resistance
superconducting
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US20100295641A1 (en
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Thierry Schild
André Donati
Armand Sinanna
Pascal Tixador
Stéphane Bermond
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Centre National de la Recherche Scientifique CNRS
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Centre National de la Recherche Scientifique CNRS
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/006—Supplying energising or de-energising current; Flux pumps
    • H01F6/008—Electric circuit arrangements for energising superconductive electromagnets

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  • the present invention relates to a system for creating a magnetic field via a superconducting magnet intended to produce said magnetic field.
  • a superconducting magnet is formed by a superconducting coil (for example, a Niobium-Titanium composite) maintained at a temperature such that the superconducting state of the material constituting the coil is ensured (for example to 4.2 K in a bath of liquid helium at atmospheric pressure for a Niobium-Titanium composite subjected to a field typically less than 10 T).
  • the zero electrical resistance thus reached enables very high magnetic field intensities to be created within the limits of capabilities to transport superconducting materials.
  • the invention finds a particularly interesting application in the field of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI).
  • a known configuration consists of utilizing a short-circuited superconducting magnet: This mode of operation, called persistent mode, is carried out by the disconnection of the electrical power supply of the coil and the presence of a superconducting switch forming a closed circuit with the coil.
  • a superconducting switch formed by a superconducting composite coupled with a heating element is a thermal switch that has zero resistance when the heater associated with it is off, the switch is then known as “closed,” and high resistance compared to the other resistances of the circuit when the heater is turned on, the switch is then known as “open.”
  • the resistance of the switch is that of the resistive matrix of the superconducting composite above a temperature known as the critical temperature, and is near zero below this temperature.
  • the equivalent electrical circuit thus formed is composed of the inductance of the magnet, typically several hundred henrys, the resistance of the magnet and the resistance of the short-circuit formed by, the superconducting switch.
  • the residual resistance of the magnet may be greater than the value enabling operation of the system in persistent mode.
  • FIG. 1 illustrates the electrical circuit enabling implementation of this compensation.
  • the electrical circuit 1 comprises:
  • the three branches are mounted in parallel.
  • the value of resistance R 1 is in a ratio from 10 to 1000 times the value of resistance R 2 .
  • Circuit 1 operates according to the two following operation modes:
  • the magnet may locally lose its superconducting properties and transit into a dissipative mode (“quench” of the magnet).
  • a transition implies that the latter is protected on itself (i.e., that the resistance developed in the magnet during the transition is sufficient to discharge the current in the magnet at a rate such that heating of the conductor remains limited).
  • the very high energy stored in the magnet typically over 100 MJ
  • the Joule effect that is generated may then lead to abnormal heating of the magnet and thus to a definitive deterioration of its superconducting properties.
  • a solution to this problem consists of adding an additional branch to the terminals of the magnet and power supply constituted of a protection resistance; such a circuit 10 is illustrated in FIG. 2 .
  • the electric circuit 10 comprises (elements common to circuit 1 of FIG. 1 bear the same references):
  • the cut-off member S 2 (and possibly S 3 ) is open so that coil L is discharged in resistance R 3 wherein the value is optimized to obtain a rapid discharge without deterioration of the magnet.
  • the current decay rate is then determined by the value of the protection resistance.
  • the superconductive switch S 1 in series with R 1 must be closed, i.e., at low impedance in comparison with other resistances of the circuit, in normal operation with stabilized current.
  • this same switch S 1 must be open (i.e., at high impedance in comparison with other resistances of the circuit (R 1 , R 2 , R 3 )) during the charge/discharge of the coil L and during the protection of the magnet (rapid discharge of coil L in R 3 ).
  • a resistance R 1 presenting a value much less than the protection resistance R 3 and a value greater than that of R 2 must be utilized (in a ratio from 10 to 1000 times the value of resistance R 2 , as already mentioned above).
  • resistance R 1 must be such that the Joule power dissipated in the stabilization regime remains low, typically less than several milliwatts.
  • a known solution consists of replacing the superconducting thermal switch with a mechanical switch.
  • a configuration of this type is described in document US2007/0024404.
  • This solution provides a provisional technological response to the second and third difficulties mentioned above but leaves the first difficulty unresolved, connected to the fact that the reliability of the magnet protection depends on the reliability of the switch and its control circuit.
  • the present invention aims to provide a system for creating a magnetic field aiming to be free from the three difficulties mentioned above while ensuring an effective charge of the coil, very low drift of the magnetic field in time and rapid discharge without deterioration of the magnet in case of quench.
  • the invention proposes a system for creating a magnetic field including:
  • the limiter must have an inductance that is as low as possible, on the one hand to ensure the stabilization function as described in U.S. Pat. No. 6,624,732, and on the other hand to minimize the transition time between the “closed” state and the “open” state. With the experimental devices utilized, it is on the order of several microhenry.
  • the superconducting material is chosen such that its critical temperature is greater than the temperature of the medium in which it is placed.
  • the temperature of the superconductor wire forming said limiter passes by a maximum value called T max .
  • T max a maximum value
  • This value must be such that the limiter is not deteriorated if the superconductor wire constituting it reaches, locally or in totality, the value T max .
  • This must at least be less than the temperature from, which the superconducting properties of the superconductor wire chosen are not deteriorated, for example around 300° C. for NbTi.
  • T max the temperature from, which the superconducting properties of the superconductor wire chosen are not deteriorated, for example around 300° C. for NbTi.
  • a T max of less than 100 K is chosen since below this value, most materials are no longer deformed under the effect of a temperature variation.
  • Superconducting limiter is understood to refer to a device based on the transition of superconductors between a non-dissipative state (near zero resistance) and a dissipative state (non-zero resistance). This superconductor transition is particularly characterized by the presence of a critical current, beyond which the device switches into a dissipative state.
  • the limiter according to the invention is distinguished from limiters intended for electrical distribution networks where the current limitation necessities only last several hundred milliseconds. On the other hand; with reference to the invention, the operation in limitation must be able to last several minutes or even several hours. The thermal exchanges that were disregarded in this type of application here gain great importance.
  • the superconducting switch controlled by a heater system according to the prior art is advantageously replaced by a superconducting limiter not necessitating any external control to switch into resistive mode during coil charge or discharge or during rapid discharge.
  • a superconducting limiter not necessitating any external control to switch into resistive mode during coil charge or discharge or during rapid discharge.
  • Such a configuration presents a considerable advantage in terms of operation security inasmuch as the effectiveness of the rapid discharge in case of quench is no longer dependent on opening the switch controlled by its external control;
  • the limiter according to the invention intrinsically allows switching from its conducting state to a resistive state during three modes of operation that are the charge or discharge of the coil, the normal operation mode and the rapid discharge of the coil in the protection resistance upon detecting a magnet quench.
  • the system according to the invention may also present one or more of the characteristics below, considered individually or according to all technically possible combinations:
  • Another object of the present invention is a method of adjusting the current in a magnet included in a system according to the invention comprising the following steps, considered in any order:
  • the method according to the invention comprises a step of generating a current slot that follows the step of generating said current pulse, the value of the current in this slot being equal to the sum of the current circulating in said protection resistance and of the current circulating in said limiter when it is in its high-resistance state.
  • FIG. 1 is a schematic representation of a first circuit according to the prior art
  • FIG. 2 is a schematic representation of a second circuit according to the prior art
  • FIG. 3 is a schematic representation of a system according to the invention.
  • FIG. 4 is a schematic representation of a system according to the invention incorporating control means to cause the limiter to switch from its low-resistance state to its high-resistance state according to a first embodiment
  • FIG. 5 is a schematic representation of a system according to the invention incorporating control means to cause the limiter to switch from its low-resistance state to its high-resistance state according to a second embodiment
  • FIG. 6 respectively represents the evolution of the power from the power supply, the current in the magnet and the current in the limiter as a function of the time by utilizing a system such as represented in FIG. 5 .
  • FIGS. 1 and 2 have already been described with reference to the prior art.
  • FIG. 3 is a schematic representation of a system 100 for creating a magnetic field according to the invention.
  • the system 100 comprises:
  • the superconducting limiter 106 is composed of a superconductor wire formed by a plurality of elementary superconducting filaments integrated into a resistive matrix, the superconductor wire may also be constituted of the deposition of a superconducting material on a resistive substrate; Later in the description we will come back to the choice of the material for making the resistive matrix.
  • the limiter 106 is characterized by two currents: The limitation breaking current Io and the recovery current Ir.
  • the breaking current represents the current beyond which the limiter develops high resistance that limits the current. This current is close to the critical current Ic characteristic of the superconducting material and is defined by the current for which the conductor develops a given electrical field (10 uV/m or 100 uV/m).
  • the recovery current is the thermal equilibrium current of the conductor with its environment. This current is reached after a rather long time (on the order of some seconds) and is not a conventional limiter parameter. It is defined by the characteristics of the conductor, particularly its resistance per unit length, and the cooling conditions (thickness of the insulator surrounding the limiter and thermal conductivity of the limiter).
  • Step 1 The value of resistance.
  • R′ 1 is defined as a function of the value of residual resistance R′ 2 of the magnet in a ratio from 10 to 1000.
  • Step 2 so as to not cause the limiter 106 to transit to a high impedance during normal operation mode, a superconductor wire is chosen presenting a critical current I c greater than (R′ 2 /R′ 1 ) I op .
  • Step 3 as we already mentioned above, the maximum temperature T max seen by limiter 106 is produced during the rapid discharge phase of the magnet 102 in the protection resistance R′ 3 .
  • the sizing of limiter 106 requires the choice of this maximum admissible temperature, W max , on the limiter 106 in case of discharge of the magnet 102 .
  • Step 4 It is important that limiter 106 does not exchange too much energy with the helium bath, particularly during magnet 102 charging and discharging operations with power supply 103 . Consequently, sizing of limiter 106 also requires the choice of the maximum admissible power on the cryogenic bath, W max , during magnet charging and discharging operations.
  • Step 5 This step aims to calculate the length of the wire that is strictly necessary to maintain the wire at a temperature less than the W max temperature set at step 3 (during the rapid discharge of magnet 102 ).
  • the voltage at the terminals of the magnet 102 , U(t), and thus of limiter 106 is provided by the following relation:
  • Step 6 This step aims to determine the thermal insulation necessary on limiter 106 to limit the power deposited on the bath. This insulation is characterized by the thermal flux per wire unit length, w insulation , between the helium bath and the limiter 106 once the steady state is established. During charges and discharges of magnet 102 , the voltage at the terminals of the limiter is constant and imposed by power supply 103 , U Alim . The thermal equilibrium between the bath and the limiter is thus written
  • the limiter is, for example, composed of a superconductor wire formed by a plurality of elementary filaments in niobium-titanium (NbTi) in which the transition temperature is equal to 9.5K if it is subjected to zero magnetic flux density and in which the diameter is preferentially less than 120 mm integrated into a resistive matrix.
  • the resistive matrix is preferably highly resistive so as to reduce the length of the wire (as we mentioned above, the maximum wire length is inversely proportional to the resistivity of the wire and its matrix): A highly resistive matrix thus reduces the bulk of the limiter.
  • the matrix may, for example, be made from cupronickel (CuNi).
  • a high Tc type superconducting material such as magnesium diboride (MgB 2 ) or a ceramic type superconductor, such as YBaCuO, for example.
  • a solution consists of utilizing a coil in two layers, the two layers being wound in opposing directions (two coils of the same length interlinked and separated by in insulator to avoid dielectric breakdown between the two coils).
  • the two layers are placed in parallel at each end: This configuration is interesting since it distributes the voltage over a large distance (the distance between the two ends) and prevents dielectric breakdown.
  • the two layers are placed in series.
  • Step 1 Choice of the stabilization resistance R′ 1 at 1 m ⁇ to ensure a R 1 /R 2 ratio of 100.
  • Step 2 Choice of an uninsulated superconductor wire with a diameter of 0.2 mm composed of superconducting filaments in NbTi of 30 um in diameter in a CuNi matrix with 30% Ni by weight.
  • the ratio of the section of Cu on the section of NbTi is 1.2 which ensures a critical current greater than (R′ 2 /R′ 1 )I op , or 4 A.
  • Step 3 Choice of the maximum admissible temperature T max at 100 K.
  • Step 4 Choice of the maximum admissible power on the cryogenic bath W max at 1 W.
  • Step 5 By applying relation 1, a maximum wire length of approximately 250 m is found. As we have already specified, this value is greatly increased; thus, tests demonstrate than a 50 m length is sufficient.
  • Step 6 The limiter is insulated from the helium bath, for example, with an insulating resin (epoxy, for example) presenting a thickness of 1 mm. If necessary, the thickness of the insulating layer may be increased in order to reduce the power dissipated to a value of less than the desired threshold value W max in steady state with the limiter in its high impedance state.
  • an insulating resin epoxy, for example
  • the invention applies to both a configuration in which the magnet 102 and the limiter 106 are in the same cryogenic bath and to a configuration in which magnet 102 and limiter 106 are in separate baths; in the latter case, a possible application consists of utilizing two helium baths, one containing superfluid helium at a temperature of between 1.7 and 2.2 K (on the order of 1.8 K) for the needs of magnet 102 and the other containing liquid helium at 4.2 K, the two baths being interconnected by a channel of reduced section according to the “Claudet bath” principle. Such a configuration allows easier access to limiter 106 separated from magnet 102 .
  • a first solution consists of adding a heater allowing the limiter to be placed temporarily in “open” mode, without necessarily degrading the security connected to the intrinsic operation of the limiter.
  • a second solution consists of injecting via the magnet current leads (in the coil of the magnet and the protection branches situated in parallel with cut-off members 104 and 105 ), a sinusoidal alternating current or impulsive current that overlaps the running current.
  • the frequency of this current is chosen sufficiently high so that the alternating current is blocked by the inductance L′ of the coil, such that the latter does not receive thermal energy likely to cause it to transit outside of the superconducting state.
  • the frequency may, for example, be chosen so that more than 99.9% of this alternating current passes through the limiter.
  • the transition of the limiter from its low impedance state to its high impedance state is obtained either by the elevation of the temperature driven by the circulation of said alternating current (elevation created by losses induced by the alternating current) or because the effective value of the alternating current exceeds the value of the breaking current of the limiter.
  • a frequency equal to or greater than 50 Hz suffices for known applications.
  • This alternating current may be generated by internal specific circuits designed for this purpose, or even externally to the power supply by a secondary power supply preferably situated in parallel with the main power supply. However, it is not contrary to the invention to produce this secondary power supply by a device placed in series with the main power supply.
  • An example of a system 200 for creating a magnetic field according to the invention incorporating a control device 201 generating such a signal is illustrated in FIG. 4 .
  • System 200 is identical to system 100 of FIG. 3 with the difference that it comprises the control device 201 forming means for switching limiter 106 from its low-resistance state to its high-resistance state by enabling the generation of a sinusoidal current signal able to cause limiter 203 to switch and that it does not comprise a second redundant cut-off member 105 .
  • the control device 201 comprises:
  • a resistance R′ 2 of 10 ⁇ (resistance simulating the resistive connections of a superconducting magnet) is mounted in series with coil L′.
  • Switch 203 being closed, the control device 201 is implemented by the closing of switch 202 (connection to network 230V/50 Hz).
  • the autotransformer 204 is adjusted to a voltage of 230 V.
  • limiter 106 transits since the short circuit current Icc (corresponding to the effective value of the sinusoidal current provided by the ELV transformer 205 ) is greater than the breaking current necessary for causing limiter 106 to transit.
  • Icc the short circuit current provided by the ELV transformer 205
  • limiter 106 In a second phase, limiter 106 being resistive, the current traversing it is weak (some tens of mA) and the voltage necessary to maintain the transited limiter 106 is therefore some volts (approximately 1 V in output of autotransformer 205 ). This voltage will make a current of approximately 2 A circulate in the discharge resistance R′3 and a very weak alternating current circulate in the mesh of the coil, inversely proportional to its inductance L′. The alternating current does not modify the main direct current in the coil.
  • a third phase one may increase (or reduce) the main current in the coil by modifying the current provided by power supply 103 .
  • the switch 203 is either closed to maintain limiter 106 open or open (in this case, the current that maintains limiter 106 open is provided by power supply 103 for the time necessary to modify the current).
  • the open switch 203 enables adjustments in current to be done without being disturbed by alternating signals.
  • limiter 106 In a fourth phase, as soon as the necessary adjustments have been made, limiter 106 becomes superconducting again following the opening of switch 203 . In fact, without the provision of external energy, limiter 106 typically finds the temperature of the cryogenic bath after some seconds. The time to return to the closed state depends above all on the level of thermal insulation between the limiter and the cryogenic bath.
  • FIG. 5 illustrates the implementation of such a control on a circuit 300 substantially identical to circuit 100 from FIG. 3 (with the difference that it does not comprise switch 105 ).
  • the circuit 300 presented in FIG. 5 is comprised of superconducting magnet of inductance 0.68H giving a nominal magnetic field of 7 T for a current I 2 of 400 A.
  • the resistance R′ 2 simulating the resistive connections of the superconducting magnet mounted in series with coil L′ has a value of 10 ⁇ .
  • a power supply 103 (1000 A-10 V) regulated in current is connected to the charge by closing switch 104 .
  • resistance R′ 3 (here of a value equal to 0.5 ohm) is mounted in parallel to the branch of the magnet.
  • resistance R′ 3 is inside cryostat C.
  • switch 104 is open, leading to the rapid discharge of energy from the magnet in the protection resistance R′ 3 .
  • Limiter 106 and stabilization resistance R′ 1 (here equal to 1 m ⁇ ) are mounted in parallel on the magnet.
  • Power supply 103 comprises means to generate a current pulse for a sufficient duration (here >5 ms) and amplitude Ip (here >40 A) greater than the breaking current enabling limiter 106 to switch from its low-resistance state to its high-resistance state.
  • a solution to generate this pulse consists of intervening in the control loop of power supply 103 .
  • Power supply 103 regulated in current, generates a current ramp (with a di/dt here chosen of between 2 and 10 A/s). A minimum ramp value is imposed so that the voltage Uc at the terminals of the magnet is sufficient to maintain limiter 106 in its resistive mode.
  • This current slot will have the same duration as the rise ramp (i.e., corresponding to the magnet adjustment time).
  • FIG. 6 represents the evolution as a function of time of respectively the current from power supply 103 , the current in the magnet and the current in limiter 106 .
  • Current and time scales are the same for the three curves. The following steps may be distinguished:
  • the delay is significant (approximately 3 s) between the start of the ramp and the pulse; this delay only sets out to illustrate the theory of operation but may be reduced to zero.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Containers, Films, And Cooling For Superconductive Devices (AREA)
  • Emergency Protection Circuit Devices (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Audible-Bandwidth Dynamoelectric Transducers Other Than Pickups (AREA)
  • Hard Magnetic Materials (AREA)
  • Magnetic Bearings And Hydrostatic Bearings (AREA)
US12/742,296 2007-11-12 2008-10-27 System for creating a magnetic field via a superconducting magnet Active 2029-02-23 US8174803B2 (en)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
FR0758969 2007-11-12
FR0758969A FR2923648B1 (fr) 2007-11-12 2007-11-12 Systeme de creation d'un champ magnetique via un aimant supra-conducteur
PCT/FR2008/051937 WO2009063150A1 (fr) 2007-11-12 2008-10-27 Systeme de creation d'un champ magnetique via un aimant supraconducteur

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EP (1) EP2220658B1 (fr)
AT (1) ATE505801T1 (fr)
DE (1) DE602008006265D1 (fr)
FR (1) FR2923648B1 (fr)
WO (1) WO2009063150A1 (fr)

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US9612308B2 (en) 2008-06-20 2017-04-04 Weinberg Medical Physics Inc Ultra-fast magnetic field for electron paramagnetic resonance imaging used in monitoring dose from proton or hadron therapy
US9411030B2 (en) 2008-06-20 2016-08-09 Weinberg Medical Physics Llc Apparatus and method for decreasing bio-effects of magnetic gradient field gradients
JP6262417B2 (ja) * 2012-07-31 2018-01-17 川崎重工業株式会社 磁場発生装置及びこれを備える超電導回転機
US9726738B2 (en) 2013-06-21 2017-08-08 Weinberg Medical Physics Inc. Energy-saving method of generating time-varying magnetic gradients for use in MRI
US9638774B2 (en) * 2013-08-05 2017-05-02 Shahin Pourrahimi Discharge controlled superconducting magnet
WO2015072001A1 (fr) * 2013-11-15 2015-05-21 株式会社日立製作所 Aimant supraconducteur
US10564238B2 (en) * 2014-12-17 2020-02-18 General Electric Company Systems and methods for energizing magnets of magnetic resonance imaging (MRI) systems
CN105118606B (zh) * 2015-09-11 2017-05-31 浙江大学 用于在线消除电磁式电流互感器剩磁的退磁电路及方法
JP6794146B2 (ja) * 2016-06-13 2020-12-02 株式会社東芝 高温超電導磁石装置
CN206498192U (zh) * 2017-02-27 2017-09-15 华中科技大学 一种基于能量快速转移的混合式直流超导限流器

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US4855859A (en) * 1987-03-30 1989-08-08 Societe Anonyme Dite Alsthom Detection device for detecting transitions to the normal state in a superconducting winding, in particular for generating electricity, and a protection device for protecting such a winding
EP0299325B1 (fr) 1987-07-17 1991-12-18 Siemens Aktiengesellschaft Aimant supraconducteur activement blindé d'un appareil tomographique à spin nucléaire
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WO2009063150A1 (fr) 2009-05-22
US20100295641A1 (en) 2010-11-25
DE602008006265D1 (de) 2011-05-26
EP2220658A1 (fr) 2010-08-25
ATE505801T1 (de) 2011-04-15
FR2923648A1 (fr) 2009-05-15
FR2923648B1 (fr) 2009-12-18
EP2220658B1 (fr) 2011-04-13

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